Laser processing has many applications. One application is laser shock processing. Laser shock processing (also known as the Laser Peen® process, laser shock peening, or laser peening) is an innovative surface treatment for improving the fatigue strength and damage tolerance of various workpieces, typically including metal parts. Laser shock processing drives high amplitude shock waves, generated by high intensity laser pulses, into a material surface. The shock waves are used to develop deep compressive residual stresses in the surfaces of fatigue-prone parts. Typically, these compressive residual stresses penetrate five to ten times deeper than conventional metal shot peening. These compressive residual stresses inhibit the initiation and propagation of fatigue cracks.
Another application includes laser bond inspection (LBI). LBI may be used for non-destructive inspection of structures assembled with adhesive bonds. LBI typically involves depositing laser energy onto the front surface of a bonded article, thereby generating compression waves that reflect off of the back surface of the bonded article as tensile waves. The tensile waves provide stresses that interrogate the bond.
Another application includes laser induced bond delamination, which generally comprises laser processing a bonded structure to intentionally induce defects in one or more bonds contained in the bonded structure.
In each of these applications, before processing, an overlay coating, which may be substantially opaque to the laser beam, may be applied to the material surface being processed. An additional layer, which may be substantially transparent to the laser beam, may be applied over the opaque overlay or directly onto the material surface (i.e., no opaque overlay coating is applied). The opaque overlay may include, without limitation, tape, paint, or a liquid erosion-resistant coating as described in U.S. Pat. No. 7,268,317, which is incorporated herein by reference in its entirety. The transparent overlay may include, but is not limited to, water, water-based solutions, other noncorrosive liquids, glass, quartz, sodium silicate, fused silica, potassium chloride, sodium chloride, polyethylene, fluoroplastics, nitrocellulose, and mixtures thereof.
The laser pulses pass through the transparent overlay and strike the opaque overlay, causing it to vaporize. The vapor absorbs the remaining laser energy and produces a rapidly expanding plasma plume. Since the expanding plasma is confined momentarily between the surface of the part and the transparent overlay, a rapidly rising high-pressure shock wave is created, which propagates into the material.
These shock wave-generation processes often produce debris and vapor from target surfaces, contaminating nearby optics and sensors of the laser processing equipment. In some instances, these contaminants form a cloud that interferes with sensors and/or subsequent laser pulses. These contaminants may also damage the optics.